"Thirty Years'War, series of European conflicts lasting from 1618 to 1648, fought mainly in Germany. The struggle was initially based on the religious antagonism engendered among Germans by the Protestant Reformation, but it was later influenced by other issues, including dynastic rivalries.

Religious tensions were aggravated in Germany during the reign (1576-1612) of Holy Roman Emperor Rudolf II, as Roman Catholics persecuted Protestants. In 1608 Protestant princes and cities formed the Evangelical Union, a defensive alliance, and in 1609 Catholics founded the rival Catholic League. Conflict began on May 23, 1618, when Protestants in Prague threw two of Bohemian king Ferdinand II's ministers out a window. This act, known as the Defenestration of Prague, began a Protestant uprising that spread throughout the empire. The Bohemians then bestowed Ferdinand's crown on Frederick V, causing dissension in the Evangelical Union, because Frederick was a Calvinist. In 1620 Ferdinand, who had become Holy Roman emperor, sent a Catholic League army, led by German soldier Johann Tserclacs, graf von Tilly, to rout the Bohemians at the battle of the White Mountain, near Prague. Bloody reprisals against the Protestants followed, and Protestantism was outlawed.

The war assumed international proportions when German Protestant states sought foreign assistance against the resurgent Catholicism. Christian IV, king of Denmark and Norway, came to the aid of the German Protestants. He mobilized a large army in the spring of 1625 and invaded Saxony. In the meantime Albrecht von Wallenstein, duke of Friedland, had gathered a powerful army and entered the service of Ferdinand II. Wallenstein's mercenaries won their first victory in April 1626. In August 1626 Tilly defeated Christian's army, and Catholic forces overran northern Germany, destroying towns and villages.

Gustav II Adolph of Sweden, a zealous Lutheran, entered the conflict in the summer of 1630. Meanwhile, Tilly, who had been given command of Wallenstein's army, laid siege to Magdeburg, Germany, and sacked the city in 1631, massacring the Protestant inhabitants. Gustav routed Tilly's troops, killed Tilly, and eventually captured Munich, Germany. Faced with complete disaster, Ferdinand called on Wallenstein to command the imperial war effort. Wallenstein's army invaded Saxony in 1632. Gustav died at a battle in Lutzen, Germany, but his army forced Wallenstein's army to withdraw. In 1633 Wallenstein struck repeated blows against the Swedish strongholds in Silesia. Toward the close of 1633 Wallenstein attempted to make peace and was assassinated by his own officers. The imperial armies then defeated the Swedes at Nordlingen, Germany, in 1634. The Peace of Prague (1635) ended the third phase of the war.

In its final phase, the battle was for hegemony in western Europe between the Habsburgs and France, which was under the leadership of cardinal and statesman Richelieu. In May 1635 France declared war against Spain, the chief Habsburg dominion aside from Austria. Between 1636 and 1645 Swedish forces allied with France scored numerous triumphs, overrunning Denmark, western Germany, and Austria. The French were also successful, routing the Spanish in 1643 and destroying a Bavarian army in 1644. In 1647 Maximilian I of Bavaria concluded the Truce of Ulm with Sweden and France. Holy Roman Emperor Ferdinand III refused to capitulate, however, and fighting continued throughout the remainder of 1647. Maximilian I reentered the war on the side of the empire, but further defeats forced Ferdinand to agree to the peace.

The Peace of Westphalia (1648), which finally ended the conflict, fundamentally influenced the subsequent history of Europe. The treaty weakened the Holy Roman Empire and the Habsburgs, ensured the emergence of France as the chief power on the Continent, and delayed the political unification of Germany.